1. Field of the Invention
This invention generally relates to impact printers, and, in particular, to printing hammer assemblies for use in impact printers.
2. Description of the Prior Art
Impact printers are well known in the art. There are two major categories in impact printers: one category includes line printers which print all of the characters in one printing line at the same time so that printing is carried out line by line and the other category includes serial printers which print characters serially one after another along a printing line. In the latter category, one typical example is a wheel printer which uses a print wheel, sometimes called "daisy wheel", comprised of a hub, a plurality of spokes extending radially from the hub and various types respectively provided at the free ends of the spokes, and an impact hammer for applying an impact force to a selected one of the types upon locating the selected type by rotating the print wheel at a predetermined printing position.
Printing hammer assemblies are employed in various impact hammers, whether serial or line. Such an assembly generally includes a printing hammer having an impact surface for applying an impact force to a selected type and a driving solenoid which moves the printing hammer forward electromagnetically when energized. In such printing hammer assemblies, their printing hammers are moved back and forth at high frequencies between the home or retracted position and the advanced position where the printing hammer makes contact with a selected type thereby applying an impact force to the selected type to form an imprint on recording paper. Since the impact conditions vary depending upon from where the printing hammer starts its forward movement when driven by the driving coil, it is important that the printing hammer resides at a predetermined home or retracted position at all times before being driven to move for the next printing operation. If there is a relatively large clearance for the home position of the printing hammer, the stroke of movement of the printing hammer will vary, thereby causing the quality of printed characters to deteriorate. Thus, it is important to insure that the printing hammer can be returned precisely to the same home position at all times.
One example of a prior art printing hammer assembly applied to a wheel printer using a printing wheel is illustrated in FIG. 1. As shown, the printing hammer assembly includes a printing hammer 10 having a hammer shaft 2 at its forward end and an armature 3 at its backward end, and the printing hammer 10 is supported by a pair of front and rear bearings 12 and 14 so as to be slidably movable in a reciprocating manner linearly. The assembly also includes a front yoke 22 which has a front cylindrical section 6a to which the front bearing 12 is fixedly mounted. A driving solenoid 24 is provided as wound around a spool 26 which, in turn, is fixedly mounted as housed in the front yoke 22. The solenoid 24 is so disposed that it can interact with the armature 3 electromagnetically when energized.
A rear yoke 28 is tightly fitted into the rear end of the front yoke 22 and the rear bearing 14 is fixedly attached to the rear yoke 28, and, thus, the front and rear yokes 22 and 28 are so combined to establish a magnetic circuit. Also provided in the assembly is a cover 30 which encloses the front and rear yokes 22 and 28. A rubber damper 20 is disposed as supported by the cover 30 at the location opposite to the rear end of the printing hammer 10, and a metal plate 18, which is a thin rigid member such as a washer, is fixedly attached to the front end surface of the rubber damper 20 thereby defining a retracted end position by the front end surface of the metal plate 18. As shown in FIG. 1, since a coil spring 16 is provided as extended between the armature 3 of the printing hammer 10 and the front bearing 12, the printing hammer 10 is located at its home or retracted position with its rear end surface abutting against the metal plate 18 when the driving coil 24 is in deenergized state. Also provided in the assembly of FIG. 1 is a protector 32 as fixedly attached at the mouth of the front cylindrical section 6a.
In operation, when the driving solenoid 24 is energized, there is produced a magnetic flux passing through the front yoke 22, printing hammer 10 and rear yoke 28 so that the printing hammer 10 is driven electromagnetically to move forward against the force of the compression spring 16 as indicated by the arrow A. Thus, the front end, defined as impact surface, of the printing hammer 10 applies an impact force to a selected type 36 of a print wheel 34 located at a predetermined printing position, and, therefore, the type 36 is strongly pressed against recording paper 42 placed around a platen roller 40 with an ink ribbon 38 sandwiched therebetween. Thus, an imprint of the type 36 comes to be formed on the paper 42. Upon deenergization of the solenoid 24, the printing hammer 10 returns to its home position as receiving the recovery force from the spring 16 until its rear end surface hits the washer 18.
The rubber damper 20 is provided for the purpose of absorbing the shock energy of the printing hammer 10 when it returns to its home position with the aid of the recovery force of the spring 16 in order to prevent the printing hammer 10 from rebounding, and it is typically comprised of a low elastic rubber material. A main objective of provision of the washer 18 is to prevent the rear end surface of the printing hammer 10 from being adhered to the front end surface of the rubber damper 20 because the rear end surface of the printing hammer 10 is normally kept pressed against the rubber damper 20 under the force of the spring 16.
However, in the prior art printing hammer assembly as described above, accuracy in positioning the printing hammer 10 at its home position is relatively poor due to several causes. For example, it is rather difficult to precisely control the dimensional accuracy of the rubber damper 20 such as molding accuracy and positioning accuracy. Further, the rubber damper 20 tends to deform due to aging and other environmental conditions thereby causing the retracted position to shift. Moreover, since rubber is severely affected by deterioration in durability due to aging, performance tends to fluctuate and thus operation is not trustworthy. Such irregularities in retracted position of the printing hammer will produce irregularities in the level and timing of an impact force to be applied to a selected type, and, therefore, resulting imprints will be poor in quality. Such a tendency will be made more noticeable if printing speed is desired to be increased.